1. Field of the Invention
The present invention relates generally to printed circuit board mounting schemes and, particularly, to printed circuit board mounting and locking schemes for use, for example, in pushbutton type radio systems of the type wherein pushbuttons on a faceplate panel of an enclosure are used to actuate switches mounted on a printed circuit board disposed within the enclosure in spaced, parallel alignment with the faceplate panel.
2. Description of the Prior Art
In prior art circuit board systems, it is well known to mount conventional momentary-action switches on circuit boards, and then to fixedly mount the circuit board in the enclosure in spaced, parallel alignment with the faceplate panel. Actuating devices, such as pushbuttons are mounted on the faceplate panel and are mechanically connected to the switches on the circuit board.
To operate a switch, e.g., an ON/OFF switch, the pushbutton is manually pressed a first time to make a circuit and pressed a second time to break the circuit. Each manual depression of the pushbutton causes a straight line compressive force to be translated from the pushbutton to the circuit board through the switch. This means of actuating the switch requires the circuit board to be fixedly mounted within the enclosure. The circuit board should be positively secured in position so that no movement is allowed at support points. If any movement occurs, the tolerances of switch activation increase and require greater travel of the pushbuttons. Generally, the circuit board in such arrangements is secured to the enclosure by means of fasteners such as screws and other board support means.
An illustrative environment where application of the principles of the present invention is particularly advantageous is in circuit board enclosures for electrical instrument circuits such as, for example, the "Protective Case for Electrical Instruments on Circuit Boards," U.S. Pat. No. 4,092,698 of Brefka dated May 30, 1978. There, an electrical instrument is enclosed in an injection molded housing having a front opening, four side walls and a rear wall, The front opening is for inserting a circuit board therein. In order to provide for mounting the circuit board, four triangular shaped corner ledges are molded along with the rest of the structure with upper surfaces positioned to support a rear-surface of the circuit board. The front surface of the circuit board is engaged by bevelled front faces at the ends of opposed pairs of resilient fingers which extend with an inward cant from opposite interior side walls into the interior of the structure. The bevelled front face of each resilient finger includes a succession of steps or ridges spaced closely together forming a stepped front face that is capable of constraining the circuit board between the ledge and the stepped face.
As the circuit board is introduced into the housing, the fingers flex away from the edge of the circuit board. When the circuit board is against the ledge surfaces, the clearances between the lower edge of the bevelled front faces and the circuit board will permit the resilient fingers to flex inwardly to bring the vertical stepped front faces into engagement with the circuit board thereby latching the circuit board into the housing with some clearance which will likely allow slight circuit board movement.
Such an arrangement permits the circuit board to be latched in the housing automatically upon its introduction therein, without resort to separate fastening means or tools. However, such an arrangement is suitable for pushbutton and switch systems that are high travel systems wherein the switches may operate satisfactorily even though there is "play" or movement of an end of the circuit board supported between the support ledge and the best-fit step of the plurality of steps in the front face of each of Brefka's resilient fingers. Switch systems that are low travel systems; i.e., switches that operate with very little or no pretravel of the plunger, cannot tolerate a loosed fit of the ends of the circuit board between the support edge and the stepped face. The circuit board must be securely held with no movement allowed in order to prevent adding any pushbutton and switch pretravel distance due to board movement.
Meshing, with a high degree of accuracy, the step gradations on the front faces of Brefka's resilient fingers with the end portion of the circuit board is a formidable task. A space gap will undoubtedly exist between the circuit board and the best-fit stepped face as the circuit board rest on the ledge. This gap is a result of the circuit board being of a smaller thickness than the distance provided between the ledge and the best-fit step on the bevelled face. If the technique of increasing the number of steps to minimize the gap is employed, each increase in the number of steps will either reduce the step length or will increase the overall length of the bevelled stepped face. Both such conditions are usually undesirable for molded latch enclosure systems due to excessive stress at the flexing pivot points of the fingers or due to an unreliable latch overlap of the step.